9.10: Alkynes to Carboxylic Acids: Oxidative Cleavage
Alkynes undergo oxidative cleavage in the presence of oxidizing reagents like potassium permanganate and ozone. The triple bond — one σ bond and two π bonds — is completely cleaved, and the alkyne is oxidized to carboxylic acids. When warm and basic aqueous potassium permanganate is used as an oxidizing agent, alkynes are first converted to carboxylate salts via an unstable α-diketone intermediate. Further, a mild acid treatment protonates the carboxylate anions generating free carboxylic acid molecules. When an alkyne is subjected to ozonolysis, an ozonide intermediate is formed, which is then oxidatively cleaved through hydrolysis to yield carboxylic acids.
Oxidative cleavage of internal alkynes yields only carboxylic acids, while terminal alkynes generate carbon dioxide, in addition to an acid, irrespective of the oxidizing reagent used. Thus, oxidative cleavage can be used to locate the triple bond in unknown alkynes. The carbonyl groups in the products are a key to determine the position of the oxidatively cleaved triple bond in the reactant: if the identity of the acids is known, the structure of the unknown alkyne can be deduced.